1. Field of the Invention
This invention relates to zoom lenses, and more particularly to wide angle high range compact zoom lenses suited for still cameras and video cameras.
Description of the Prior Art
In the field of still cameras and video cameras, a demand exists for zoom lenses of wider image angles and increased ranges. In general, an increase in the image angle of the zoom lens produces high order aberrations which are difficult to correct. An increase in the varifocal range requires an increase in either or both of the refractive power of each lens component and the total zoom movement. This leads to an increase in the diameter of the front lens component, as well as in the total length of the zoom lens. Therefore, it is difficult to minimize the bulk and size of the entire system.
Japanese Patent Publication No. SHO 44-14877 discloses a zoom lens which is constructed, from front to rear, of positive, negative, positive and positive lens units. When zooming from the wide angle to the telephoto end, the first and third lens units are moved axially at the same speed, while the second unit is moved axially in non-linearly differential relation. This zoom lens, because it is of the telephoto type, has a relatively narrow maximum angular field of view of 38.3.degree..
Japanese Laid-Open Patent Application No. SHO 54-30855 discloses a zoom lens of this type but with a widened maximum angular field of 64.degree.. However, this zoom lens has a zoom range of about 3 times, which is not always satisfactory.
Attempts have been made to extend the range while preserving good stability of aberration throughout the extended range by increasing the number of zoom components. Such a system is disclosed in Japanese Laid-Open Patent Applications Nos. SHO 57-168209 and SHO 57-169716, wherein the zoom lens is constructed of four lens units of positive, negative, positive and positive powers, from front to the rear, all of which are movable for zooming. The arrangement of the refractive power of these four lens units is such that the paraxial pencil of rays emerging from the third lens unit is parallel. The use of such a refractive power arrangement tends to give the lens a relatively long total length. By converging the paraxial pencil of rays, the total length of the lens shortens, but the range of variation of aberrations with zooming increases. In general, therefore, it is very difficult to achieve good stability of aberration correction throughout the extended range.